I just installed the JB4 stage 2 on my 2013 535xi and so far it feels pretty damn sweet. Right off the bat you can feel the extra power. Feels like the car shed a few hundred lbs when it accelerates now. This is at the default 3.5psi over stock setting.
Didn't get to romp on it the car much but so far just normal driving around feels alot nicer than stock.
The install was a breeze. Took me alittle while just to get familiar with the N55 motor but i can probably do the install again in less than 10mins
My problem now is finding a good spot to run the cable/jb4 so it looks more stealth.
Current - 2013 N55 535xi Msport SAT
Retired - 2009 N54 335i Msport 6MT
Retired - 2000 M52TU 323i ASA Supercharged 5MT
More photos of my car here